Method for foaming synthetic resin bodies with microwave or high frequency energy

Abstract

A slab of rigid heat-foamable organic synthetic resin is heated until at least a portion of its surface is at or above the softening temperature of the resin, whereupon a microwave or high frequency field is allowed to act on the slab until foaming occurs.

What is claimed is: 
     
       1. A method for making a rigid foam article, which method comprises heating a heat-foamable rigid slab of synthetic resin containing a blowing agent until at least a portion of the surface of said slab is at least at the softening temperature of said resin but foaming is not initiated, and thereafter subjecting the warmed slab to microwave or high frequency radiation, whereby said slab is further heated to expand said blowing agent and to foam said foamable slab, wherein said synthetic resin comprises an acid monomer selected from the group consisting of acrylic acid and methacrylic acid and a nitrile monomer selected from the group consisting of acrylonitrile and methacrylonitrile, said acid monomer being at least 20 percent by weight of said polymer and said acid and nitrile monomers together being at least 80 percent by weight of said polymer. 
     
     
       2. A method as in claim 1 wherein the ratio by weight of said acid monomer to said nitrile monomer is from 3 : 2 to 2 : 3.